I present to you the Freight Vac!

A hot tub or pool vacuum made with inexpensive components. Designed for use with the saluspa line of inflatable hot tubs, this vacuum can be used to suck up debris and sediment buildup in and around the hot tub interior. Designed with a buoy to remain vertical, the vacuum floats along side you and discharges clean water from the outlet above the water line.

It utilizes a cheap fluid transfer pump ($13 from Harbor Freight), a filter (same as the hot tub itself), and a dash of silicone to seal.

Assembly:

1. Print parts
2. Remove hose ring, hose, and inlet ring from existing transfer pump. Retain hose for future use. Hose can usually be removed by hand, inlet ring requires a tool to remove. I used a pocket knife, but a flat head may suffice. Once remove, it can be discarded.
3. Apply bead of silicone to canister. There is a groove where the lid meets the canister. Silicone must fill recess, wipe away excess to be flush with surface. Allow to cure before use.
4. Insert transfer pump into port. Press firmly. The retaining ring mimics the original, and will snap onto the groove.
5. Apply bead of silicone between transfer pump and canister. This is to ensure tight seal and proper suction. Allow to cure before use.
6. Insert Saluspa (or suitable replacement) filter. Align filter recess with guide inside canister. This creates a seal, forcing water through filter, as well as maintaining equal space between filter and canister wall. Thread lid on canister, and tighten until snug. An opposing ring on the lid will create similar seal to filter, as well as an external seal against silicone on canister lip.
7. Install original hose onto inlet port of canister. Original retaining ring can be used (and is preferred). New ring included in case original is lost, missing, or broken.
8. Apply buoy halves to top of tube below outlet port of pump. Use six small zip-ties to secure. Printed with cubic infill @ 10% for additional buoyancy, this serves to keep the batteries and switch dry.
9. Insert 2 D-cell batteries, submerge in water, and start cleaning.

It is recommended to crack canister lid open underwater prior to use. This will force water to the inlet of transfer pump, allowing for use. Failure to do so will result in minimal water flow, as the transfer pump requires priming. Once full, close lid, and turn on pump. Allow 15-20 seconds for water flow to begin.

https://www.harborfreight.com/battery-operated-liquid-transfer-pump-63847.html

https://www.amazon.com/Bestway-Inflatable-Cartridge-Replacement-Accessories/dp/B08R11D8NJ?th=1

https://www.lowes.com/pd/GE-Advanced-Silicone-2-Kitchen-and-Bath-Tub-and-Tile-2-8-oz-White-Silicone-Caulk/5013491817