Some of these may be inaccurate.

Platinum Watch + Copper Wire = Detonator
Rag + Alcohol = Molotov Cocktail (add oil too for larger explosion)
RDX + C1 = C4
Cantene + Cola Can = Full canteen
Steel Pipe + Quick glue + Duct Tape = Gun barrel extender
Aluminium rod + Spring = AP enhancer (makes gun fire faster)
String + Soda Can = Alarm

Merc needs high electronics and mechanical skill for everything below.Edit

X-ray Tube + Fumble Pack + Chewing Gum = X-ray unit. 
Lameboy + Copper wire = LCD display.
X-ray Unit + LCD Display = X-ray detector. This device works like a radar, when used it displays positions of enemies not seen by your squad. It has a limited range and requires batteries.

Makeshift Mines:
-Simply take a mine/defensive/smoke/tear gas/mustard gas grenade or emergency flare.
-Attach tripwire to it.
-Voila, you made a tripwire mine out of it. You can plant it like a regular mine, but it can also be activated by surrounding tripwire.
-Click on the icon in the items picture in UDB to remove the tripwire, giving you back the original grenade and a piece of tripwire.

Attached Explosives:
- Can attach rubber band to any grenade.
- If there's rubber band attached, allow attaching another grenade.
- Can attach duct tape to any explosives.
- Can attach any explosives to duct tape.
- Can attach any tripwire activated item to tripwire.
- Use buddy item to define special attachments (like gas can, alcohol bottle or marbles).

Community content is available under CC-BY-SA unless otherwise noted.