[simcop2387]

It's not likely going to be cheaper to roll your own once you add a case and weather proofing and battery handling. These things can be had for as cheap as $15 on amazon, https://www.amazon.com/Crosse-Technology-308-1409WT-CBP-308-... or even down to $5 on aliexpress, https://www.aliexpress.com/item/1005003408554165.html

Along with that using 433MHz means that there's a much longer range, and they generally also have much lower power usage compared to doing wifi, which makes the batteries last a lot longer.