Get a cheap shop-light from the hardware store. Mine cost about $9usd. Then use a can opener to carefully cut out the bottom of a coffee can. Epoxy that side to the rim of the shop-light. Done. A strong steel epoxy works best. I used PC-7. It hardens faster with heat, so after a day, I was able to speed up the cure by just turning on the light for a bit. (It does get hot! But it's safe to work with).
Best part is you can use coffee can lids to create different sized "focus caps", or cut-out shapes. I had to line the lids with aluminum foil to prevent light seeping through, but it works pretty well.