Inflatable but plug, make one from a cycle inner tube, you can make large or small by getting different sized inner tubes.
The best way I found to make them is to:
1) Cut a section to the desired length, make sure that one end is about 2 inches past the valve.
2) Turn the section you cut inside out then using some thin tie wire seal the end away from the valve by wrapping the wire round and round then twisting the ends together to keep it really tight, fold the wire flat and cover in hot glue (from a hot glue gun)
3) Turn it back in the right way so the end you tied off is now inside the tube, ironically it will look like a sphincter on the outside where it's held tight by the wire inside.
4) Seal the other end in the same way, this end doesn go inside so the wire is on the outside, cover it in hot glue or insulation tape.
5) lube your ass
6) Attach a pump and prime so its hard enough to insert.
7) Pump, Pump, Pump until you cant take any more
NOTE: to deflate you need to remove the pump and depress the valve, this means that once inflated yo ucan also remove the pump put the cap on the valve and wear it around for as long as you can take it.
Ascii Art Diagram here: