Physically you have the right idea!
You need a drive converter from 2.5” to 3.5” like this [https://www.bhphotovideo.com/c/product/1051164-REG/icy_dock_mb290sp_b_ez_fit_pro_2.html/?ap=y&ap=y&smp=y&smp=y&lsft=BI%3A514&gclid=Cj0KCQjw0pfzBRCOARIsANi0g0vSdfxnibSrcSusQQfj5G8BQslXcgi1OLExyCj4Ib2dEimdNENvR3IaAjT5EALw_wcB|Icy Dock EZ-Fit Lite Dual 2.5" to 3.5" SSD/HDD Mounting Bracket] You’ll also need to get a drive which offers compatibility like [https://www.samsung.com/semiconductor/global.semi.static/Samsung%20SSD%20860%20QVO%20Data%20Sheet_Rev1.pdf|Samsung 860 QVO]. As we can see under the '''Interface line: ''SATA 6 Gbps Interface, compatible with SATA 3 Gbps & 1.5 Gbps interfaces''''' are all supported! Make sure what ever drive you get lists support for the older SATA I (1.5 Gb/s) standard. Here’s the needed guide: [guide|891]
You will need to make sure you’ve got the newest OS your system supports installed - OS-X Snow Leopard 10.6.8
You should also get [https://cindori.org/trimenabler/|TRIM Edabler] as Snow Leopard don’t not offer TRIM services natively.