You need a glue called Plastex3000 to bond HDPE to acrylic. It might actaully be easier to get a HDPE cutting board and route it into a flange and then bond that to the top of the tank, then acrylic riser, then acrylic neck. There is a place to get the round acrylic domes pretty cheap is you cannot make one that big, I'll dig up the link or just pm spazz and he will know off the top of his head.
Dart.....yeah I think if you find a used Dart for $100 and a Alita on a deal as well you should be very happy with that matchup. Do yourself a favor though and remote the collection cup on that bad boy so you don't ever spill gallons of skimmate down your shirt
