I think almost all the places that offer investment and you are willing to gamble are in a collective group of best places to invest. First of all you need to check a casino and see if they offer investment or not, these are a group by themselves so some of them will offer investment and some of them won't so take the ones that offer investment and put them in a box.
Now, go back and check all websites and see the ones you would not mind gambling on and the ones you wouldn't gamble and put all the ones you would gamble in a box as well. Now check both boxes and find the ones that are on both of the boxes, places you would like to gamble and places that offers investments, if there are places that exists in both of those boxes then those are the places you should be investing into.
You have a nice suggestion, at least by the time these is done one can easily conclude on the site to invest but there would be sites that accepts investment and yet no so cool for one to gamble but if the purpose of the OP is just investment, the best is to list out all the sites that accept this investment, then check the percentage offers and also read reviews to deduce the one that is most reputable among the listed options.
I am not aware of gambling sites that allows investment, since I don’t invest in gambling, I only play my games for fun but if stakes allow, then the OP can check them out because I read good reviews about them a lot of times.