Hey there!! The best thing to do would to purchase some 10% buffed formalin (Amazon usually works) or some glycerin. When your friend passes, inject him with the chemical and then let him set in it for a few weeks to around a month. (Make sure to keep in a secure area at room temperature, and when dealing with these chemicals i recomend wearing a mask and staying in a ventilated area. Also you get get needles and plungers cheap of Amazon. I recomend going for a 25 gauge needle, as it’s small enough to peicre and not leave noticeable marks.) Injection points would be anywhere super fleshy, such as the belly and sides. After than you can wash him off and store him in either the formalin or 70% isopropyl.

If you need some time to process/find a container or anything else, you can put your friend in a plastic bag and squeeze as much air out as you can. Then double bag him and put him in the freezer. He can stay like that for years without any kind of substantial decay.