I am an aquaclear nut! I have all aquaclear models except the AC-300. I can't tell you if it would fit or not? I have found with smaller tanks , sometimes the thickness of glass or acrylic is so thin you need to place a shim between filter and tank to make level for proper operation.
Their is a filter made just for you. I think you'll like it if you can find one. The price is right too! Its not an aquaclear. This filter is an AZOO PALM FILTER This is a mini filter (HOB) 3"x 3 1/2"x 5" filters 16 GPH is recommended for aquariums up to 5 gallons. This filter is $6.49 at www.drsfostersmith.com replacement filter pads are $1.79 for 4 pack.
I have seen this filter set up at my local fish store, they sell them for $10 . They have them set up on small tanks like yours. Owner says they work great, He of course wants to sell one too?
I suggest looking for one at a local store where you can see it in operation for yourself, before you buy. Just because I liked them doesn't mean you will. Just an idea?