An inflatable buttplug, which comes relatively cheap.

1/A piece of garden-hose with a balloon inside
2/A clamp
3/A valve-connector for compressed air
4/Another clamp
5/Another piece of garden-hose

The idea is quite simple
The valve-connector is airtight, when you remove the smaller part. This is also the most expensive piece.
Above you see the buttplug inflated, but of course it needs to be 'in place' before inflating it.

Here I've pulled a 'bite-ring' down over the hose, in order to protect the anus from the clamp, which may have sharp edges. They come in sets of 2. I.e. www.ingdams.com
Another option is to wind a good layer of elastic tape around the clamp, either as a replacement of the ring, or in completition of the protection - as wounds in the anal area have a tendency to a/ get infected and b/ heal slowly !

Here you see the buttplug, ready to be inserted ( but for the fact, that the clamp is still un-protected/unwrapped.
I've powdered the balloon ( a giant one) with talc, and stuffed it down into the garden-hose, in order to use the garden-hose to pass the balloon through the anus. Apply a good portion of a waterbased lubricant to the hose before ... AND: blow up the balloon with great precaution !