I used a piece of offcut mdf which cost all of 60p, an opened out cereal packet to make a series of templates, some sticky-backed plastic and left over fabric. The filling is made out of ground up polystyrene packaging. The only specialist tool used is a staple gun.
Step 1: Templates
Make a sketch first of all so that you can see the shape you want to end up with - this way you can see the outline you're aiming for rather than try to visualise it in stages.
First I lined up a long edge of the card with a narrow edge of the board, using one of the creases in the card for alignment. I used the coaster to make circles to correspond with the front and back corners of the finished shape - so the front one needs to be an inch or two in from the narrow edge, and the back one needs to be similarly in from the the side and back edges. I drew a freehand curve to join the circles, and cut around the shape, going halfway around each of the corner circles, and cutting straight out to the short edges. Mark one of the corners, either front or back, so you don't get mixed up.
Using the crease in the card for alignment, draw around the shape for the first end, flip the card over, and draw around the shape for the other end.
I did the concave, front edge next. I used the other long edge of the card for this. My piece of card wasn't as wide as the board, so I had to make alignment marks on the board to correspond with each end of the card. Next I marked the centre of the card - fortunately this corresponded with a fold line.
First I laid the card on the board, then used the coaster to line up with the part-circles already drawn, and continue these lines on the card. I then drew a freehand curve on the card between the centre and one coaster circle to create a nice shape.
Next I cut out this half shape, folded the card in half and drew along the cut half, to create a symmetrical pattern. Then I cut out the other half.
Using the alignment marks, I used the template to draw the front edge on the board. Without the template, I tidied up the line freehand.
I lined up the middle of the concave shape from the front edge template with the back edge of the board, still keeping the card between the alignment marks, then drew a freehand line from one of the coaster circles to the centre of the card, and cut away the piece (this won't work if you try to use the section you made for the end). Again, I folded the card, drew around the half section, cut the other half, drew around the template and tidied up the line freehand.
Step 2: Cutting Out the Shape
Step 3: Cover the Board
Lay the sheet face down on a flat surface and peel off the backing, then lay the board on top of it ensuring it's reasonably centred.
Tradition is to start peeling one edge of the plastic, stick down the peeled bit and pull the backing out from underneath, but I don't think this is a good method for this project.
Make cuts into the edges of the plastic all the way round so that they appear to be radial to whatever portion of curve they're on. The distance between the cuts should be no more than 1/2". They need to be closer together on the tighter curves. The cuts should just reach the edge of the board. Pull the flaps up tightly and stick them down. I didn't cover the back of the board since it was going to have cushion fastened to it, but if you wanted to make the board without the cushion you could cut out another piece of plastic, stick it on and trim it to shape. I forgot to take a photo of this part, so I've made a diagram to illustrate how the cuts go.
Step 4: Make the Cushion
This is how I should have made it.
(Note: for non-sewing people, by a hem's width, I mean whatever amount you are comfortable leaving between the line of stitching and the edge of the fabric. Typically this would be about a centimetre or just over.)
Cut out a piece of fabric the same size and shape as the board. Once it's been stitched it'll be a bit smaller. Ideally it needs to only be the size you need to fit on your lap, however this means shaping the edge pieces to join dissimilar panels together.
Cut 4 strips to roughly correspond with the long and short edges of the tray, making the board-side edges a bit longer than the fabric panel side edges - ie, they are roughly trapezoid. Ensure there is enough for a hem at both ends of each one, so that when they are stitched together into a ring it is smaller than the panel by enough to allow a hem, and fits the board snugly at a hem's width from the other edge. It's probably better to find this size by pinning pieces longer than you need onto the panel, then cutting the short edges to shape. Use pins to hold everything together until you're sure it's right. Stitch the short edges before the long edges.
Cut a piece of fabric the same shape as the board but a hem's width bigger. I used plain calico for this.
Working wrong side out, stitch the strips together end to end so you get a slightly conical ring, then stitch the ring to the panel, leaving a gap a couple of inches wider than the zip you are going to use.
I don't know the correct method for installing a zip, but this way worked fine for me:
Position the zip face down on the right side of the fabric, with the edges together and the track of the zip against the fabric, not the gap. Adjust the position of the zip so that when the seam is complete the fold of the fabric will just cover the track of the zip. Professionally made cushions have the tag of the zip towards the inside of the cushion, so you may want to consider doing it this way. Pin, tack or stitch the zip in place - whatever makes you happy. Turn the piece and attach the other side of the zip in the same manner. Zip it up first so you don't get the sides mis-aligned. Sew the zip onto the fabric.
Sew up the rest of the edge so that the ends of the zip are hidden, then sew right through the right side, hem and zip so you get a neat little fold along the edge of the zip. Put a few stitches across the ends of the zip to strengthen the seam.
Finally, sew on the piece which will back onto the board with the whole thing inside out, then pull it the right way out, through the zip.
Step 5: Fix the Cushion
I had considered using Velcro tape in order to make the cushion removable, but decided I preferred the fastening to be as close to the edge as possible, more easily achievable by stapling direct.
Step 6: Filling the Cushion
You can buy bean bag filling, but I wanted to see if I could make my own. The end result is a bit fine, but does the job, and best of all it is free!
You need a variable speed electric drill and a wire brush attachment, some polystyrene packaging, the bags you intend to fill and a large bin bag. The large bin bag is to reduce the amount of mess you make - and believe me, there's a lot of mess.
Roll the edge of the large bag down a bit to stabilise it, put the first smaller bag in it, again rolled down a bit, and put your first piece of packaging in here. Start using your electric wire brush to strip away the packaging, so that the bits fall into the bag you want to fill. Remember that this bag has to fit through your zip and squash down so don't overfill it. Do a second bag likewise. I had enough with two but three would have made for a more even filling.
Even if you are using bought beanbag filling, the pedal bin liners are still a good way to fill the cushion. Squash the bags in through the zip and zip it up, and you're done.