There are other options i know zip tying the blocks will work as i have done some myself ,,,, However you can get something to extend the snorkel ..... how big is the outside of the snorkel ? Plastic culverts which are heavier duty then is the snorkel . If you only need a foot or two that might be tough to find some willing to cut down a section of pipe. But there are other options as well they have green pipe in every size that is also made for water lines you can get an over size piece and put a collar around your snorkel all it's doing is keeping stone from falling down the tube. they also have joints for black double walled culvert pipe that are made to put pieces of pipe together where you can use that they are easy to work with and if you need those a little tighter if you wrap a ratchet strap around it you can close it up tight against your snorkel. Then use some HD zip ties to hold it together.... any hd civil commercial supplier should be able to give you options
you could even look at something like this which also will give you security to keep kids from falling in ....
There are many other options out there that you should be able to find without breaking the bank. try manhole covers sprinklers can have wells that are like 2 feet deep and 18 inches wide with a cover that can be secured.
You could always use a larger hard pipe and use epdm and make a gasket so stop items from getting in.
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