I found a kids 16 inch bike in a dumpster, and thought it might make a good base for a adults sized scooter.

It was pretty cheap to make with most of the cost in bolts and paint.

Materials

1 child's 16 inch BMX bike
20mm plywood (800 x 160mm)
22.2mm steel tube
5 x 8mm bolts and nylock nuts
4 x 6mm bolts and nylock nuts
7 x panel washers
1 can undercoat spray paint
1 can topcoat spray paint
8 x 60mm wood screws
Brake cable inner and outer

Tools Used

Hacksaw
Saw
Rasp file
Metal file
Drill with 6 and 8mm bits
Grinder
Wire brush
Ring spanners

Step 1: Deconstruct the Bike

Remove all the parts that are surplus to your requirements, and strip the bike down to the frame.

Put aside the following:

Wheels
Brake
Headset stem and handlebars
Forks, bearings etc

Step 2: Saw the Frame

The cuts are shown in the photo.

Hacksaw off the top tube and seat tube.

Cut through the bottom bracket. Make sure the bearing cups are out of the bottom bracket as these will be harder to saw through than the bracket itself. Clean up the parts for painting later.

Step 3: Make the Deck

Make your deck long enough to get both feet on when riding. Remember a bit of room will be taken up by the frame.
Cut out a hole for the rear wheel. I made mine by using the chain stays as a guide.
I wanted the fit to be tight and ending up filing a groove out for the frame to sit in between the wheel and the deck.

Also cut out some supports to bolt the chain stays to.

Step 4: Assemble the Rear Deck

Glue and screw the rear wheel supports to the underside of the deck.
I used 4 x 60mm screws in each side. Counter sink the screws as you don't want them sticking up in the top of the deck.

Once the glue has dried, work out how you can bolt the chain stays onto the deck. Ensure you have good ground clearance
for the back wheel, but not too high to make scooting a major work out.

I ended up changing the position of the frame on the deck to give more ground clearance. The second photo shows how it was before modification.

Step 5: Front Deck

I found a 60mm pipe that matched the internal diameter of the cut bottom bracket. I bolted this to the frame with 2 x 6mm bolts. I then bolted the tube to the deck with 3 x 8mm bolts. I put big panel washers on the underside of the deck but it would be better to use a plate to spread some of the load.

It would have been great to get a plate welded onto the down tube, and just bolted that on. This is one part of the build that needs improvement as there is some flexing in the deck.

This was improved later by screwing a aluminum support under the deck.

Step 6: Extend the Handlebars

I found a 22.2mm pipe that fit snug into the forks, and a sleeve that would take the stem from the bike. This is another situation where some welding skills would be valuable.

The handlebars can be set at whatever height is comfortable.

I sourced some old grips and remounted the brake after painting. The brake cable has been extended.

This design works well enough, but I'm sure it can be improved. Good luck!