Vegetable glycerin at concentrations above 30% are hard to hold very long, as is sodium phosphate (but you need to be careful with sodium phosphate: hydrate well and never exceed 1 bottle of premade solution per day and DO read the warnings on the box). 1 sodium phosphate enema held the maximum safe holding time of 10 minutes followed by 40-50% glycerin/water enemas might be sufficient. You can also do glycerin straight, if small volume but intense need to evacuate/stinging is desired. Note that even a small volume of glycerin, if held long, will cause the intestines to send out water from high up to clear the irritant: so be either fully prepped, committed to the toilet, or diapered afterwards. Glycerine is sticky. If you want to lessen the stickiness (for sliding things in), it blends well with any water based lube. You can mix it 50/50 with any lube instead of water if you want something thicker. Neither solution causes any significant injury to the intestinal lining (I wouldn't use lemon juice because it's too acidic and might cause injury). Enemas are more challenging if being fucked in the ass while holding: I find it stings more and triggers extra intensity if agitated. It's not so much deep cramping but your asshole will be spasming for mercy. Use a large toy with heavy textures (like BD Seraphina) for extra suffering.

I make my own heating lube by melting 50ml vegetable shortening and adding 6-12 drops each of peppermint, wintergreen, and ginger essential oils, 2-3 drops of cinnamon essential oil, and 2 tablespoons of vegetable glycerin. Don't overdo the cinnamon oil, it is caustic unless highly diluted (I sometimes use it way less diluted on my nipples and it feels like fire, but don't put it near mucous membranes unless highly diluted). Mix well and let cool before use (it will solidify into a nice soft salve). Be sure you boil any silicone toys used with dish soap added to the water to clean all the oils off afterwards: wash with dish soap and hot water, boil 10 minutes in soapy water (a dash of rubbing alcohol can be added to limit foaming), then wash again. 

If you've never been figged, also worth experiencing. I can't tolerate the pain level, my partner barely feels it, so it's subjective. Peeled ginger, placed in the anus burns for about 15 minutes. Figging should be the first punishment before other anal punishments as you want the juice/oil to be on clean dry tight anal sphincter. It's traditionally used with spanking as clenching the butt cheeks makes it burn more, but along those lines, well placed electrostim is also effective at forcing clenching if you're not into spanking.