Go to the dollar store and buy a pool/swim noodle. They come in various sizes, most with a hole running through them and all about 6 feet long. Buy some large size dog collars. Look for some cheap KY lube and warming gel while you're at it. Maybe make a couple of trips so the inquisitive checker doesn't put two and two together. At a store like Walmart buy a roll of clear plastic packing wrap. It's like strong Saran wrap on a spindle that is stretchy. Get some large size ribbed and lubed condoms.
The foam noodle slices easily with a knife, you can whittle the end to make it tapered. The foam is very forgiving, you don't need to worry about sharp edges as it's so soft. Rough sandpaper will quickly smooth out the rough cuts and provide a bit of soft texture.
Wrap the wrap tightly around the end to form a nice insertable tip and work the wrap down the shaft. Find that stack of recordable CDs you have and pull off the CDs, leaving a nice round platter with a convenient shaft to insert all the way into the noodler. sometimes some tape around the shaft will make it stick in more tightly. Get the key rings and open them enough to slide through the slots at the outer edges of the CD holder bottom. Adjust the dog collars around each upper thigh. Run the key rings through the loops of the collars.
All you need now is to put one or two rubbers on your new toy, lube it up, shove it up where it belongs, and snap the collars around your thighs. Now get up and walk around. The leg action will rotate the shaft inside you. Maybe it's not so much a butt plug but a portable fucking machine!
Once you have these materials, your imagination is the limit. You can make a dildo instantly. Buy the long wide bandaging and/or gauge to wrap around and increase girth before wrapping with plastic. Wind thick string down the shaft before wrapping for pleasure ridges. shove a vibrating egg into the the end. Create a T bar by shoving a pencil through the side at the bottom. Use cord through the hole you just made to make a crotch/waste string to hold it in place.
If the object seems to large at first, remember that the foam rubber compresses and the wrap will slowly compress it, so it will get thinner with time and use. You can use rags or paper towels to pack the inner hole to add rigidity. I find some rolls of kitchen wrap have thin, strong cardboard tubes in them that fit perfectly through the noodle.
Use a heated metal skewer to melt ridges to provide extreme texture.
The main down side to this material is it's so soft it gives you a full feeling but not the hardness a stronger material would provide. Wrapping the bandaging tightly around it helps. As you get more comfortable you can peel off the rubbers and keep adding more bandaging to provide the best fit. Practice makes perfect!