I made some cup heaters for my shed, one for me and two to give as gifts to family if they worked. I've only photographed the process on the third one, when I may have got a little overconfident (and messed up one of the later steps). You get a good few hours of heat from the contraption and it actually keeps my fairly small shed pretty warm as well as providing a hot cup of tea.
Step 1: Items
You'll need at least three bolts with three nuts apiece. I made a tripod this time but used four legged designs on my other two.
The bolts are 6mm and 70mm long, I found this length gave a decent contact between the flame and the slate for the heat transfer.
The slate was an old shard from the garden (and isn't the final one, turns out triangles break more easily than squares with the sharper angles).
Step 2: Shaping the Slate and the Wood
I knocked the slate into a rough triangle with the bolster chisel then laid it on the plywood, roughly drawing around it in pencil. I sanded the edges of the wood and laid the nuts where I wanted them, marking the slate with a nail poked through the hole.
I wanted the base to be slightly larger than the top to give some stability, given the greater weight of the stone and a full teacup on top.
Step 3: Drilling the Holes
This is where it went wrong, since I pressed too hard with the drill and the first slate cracked along the middle. Luckily I had spare pieces around.
I rested the stone on the wood and drilled through the stone far enough to mark the wood to drill after.
The holes really need to match up, or at least enough to have some wiggle room until the bolts are screwed in tight.
Step 4: Screwing It Up.
And yes, they're bolts not screws but still, allow me this pun.
Once you've got the holes in, you can either fasten from the slate down or the wood up. I used the other method for the other stands but wanted to see if it worked this time. Turns out it does, so once you've slotted the bolts in, just tighten the nuts enough not to drop off (use a washer if you want to reinforce the slate around the hole) and a second nut on each one to halfway down.
Step 5: Fixing the Base.
Slot the bolts through the base plate and put the bottom nuts in place. Now you can tighten the nuts against the underside of the slate and top of the wood, you may want to put a drop of superglue on each one afterwards, I did just to make sure they didn't come loose.
This is also where I realised I'd made a mistake but I'd already applied the glue... oops.
Step 6: Tealight Holder
Yes, that was the mistake. On my other stands I'd used a piece of copper (I think from a weed sprayer but I can't be sure) and made a little fence from staples to hold the light in place but I forgot that step on this one. Instead I drilled some thin holes from the base then fitted panel pins into them, using some epoxy to hold them in place. I also glued some squares of fake leather over the bolts for the stands.
I drew around the tealight first then went slightly outside the circle to fix the fence on. Ideally you'll do this after drilling the holes for the bolts but before fastening them together. Once it's in place, the tealight should be roughly centred under the stone to give best heat absorption.
Step 7: Optional: Wall
On the other pieces I used sections of tin can snipped out and bent around the bolts to provide a wall around three of the four sides. This reflected light and kept draughts away from the flame, and maybe ensured that the heat went straight into the slate but I'm not really sure how much difference it honestly makes.
On the triangular one I just folded some tin foil to demonstrate. I'll replace it with proper tin later, when I have an empty can available.
Step 8: Finished.
That's it, the heater is done now. Light your candle and you should have a hot stone surface within two or three minutes that'll last for about fifteen minutes after the flame goes out. It really does get surprisingly hot too so be careful with it.