In aluminium you really want either a plastic or at worst stainless steel housing. Depending on how good you ally bloke is, the pic of what's available in your budget would be the 520-5PSD.
https://www.chsmith.com.au/Products/...epth-only.html
It will be the hardest to install to work at speed but is the best of the plastic 600 watt thru hulls in outright performance. Other options are the P319 from Airmar. (520-PLD)
https://www.whitworths.com.au/furuno...ull-transducer
It will be easier to install by a long shot but not as good IMO.
From there we go to the stainless units but the price goes north of $500.
https://www.chsmith.com.au/Products/...ransducer.html
or you could go rubber block and a matching box but that will smash your budget into the next century.