Also had this problem with this my Fluval Flex 9; the flow was way too strong for my guppies, nevertheless a betta.
Fortunately, had an old sponge filter laying around and was able to jury-rig it similar to @amr427. I'm fairly certain it's this exact filter, available on Amazon for less than $7 (link below): AQUANEAT Double Bio Sponge Filter (small). Unfortunately, I got it second hand though, so I can't 100% confirm...
The sponge/intake for the filter conveniently fit right into the outflow, diffusing the flow. And water is still pulled into the back chamber, at a much more reasonable rate. Cheapest and easiest option IMO; I also tried ziptying sponges over the nozzles with little success.
https://www.amazon.com/Aquaneat-Sponge-Filter-Aquarium-Accessories/dp/B08S2PDCBR/ref=sr_1_6?dib=eyJ2IjoiMSJ9.BT54MgDQ9lfxXXgJ8CWn2yeFmL3T2CESbQBLFJty8F5u1AjttiNXCcYhuJocZpiemwpv_ocmbeGwp6LKGjEX6i8cxW-fwSK6MXikQghORnKsRuO7KYXjcCN6perep9SWky9fI-56TmFZKKUsbz0Rj4zzi6Ul3mt3vyFNI-ctIb-dJsolm9LG7M4HTVeCbbDDsaGP7U6Qw5cvbeyu8F5HvRmXlNxP7yNrPr8PVgTzQMzHBzc0aSSntL3uxTmuP2loUNX6OWcF40s23GI9xoff9H2AvyVuUTQSk0Y52AB58sQ.lsA9ZKUbZ4f2mUPnZzDqmAi2kETd0DPYaUUyWG0bvS8&dib_tag=se&keywords=AQUANEAT%2BDouble%2BBio%2BSponge%2BFilter&qid=1710268161&sr=8-6&th=1
