I have two of these and they work great:
https://www.amazon.com/dp/B09WMPX1KK?ref_=cm_sw_r_cp_ud_dp_Q215ZVNTPKJH0BJHKHK4
However, at $40/each, the official Eufy ones would be a the cheaper route if you can get two for $70. I have never had to recharge the batteries in those two cams since getting those panels. They do come with the little rubber pieces on the connector to make sure moisture doesn't get into your charging port. I've had the panels for about 7 months now and no problems with them.
There are lots of small 2 wire connectors that would work. You aren't providing high currents so something like shown in the link below should work.
Just got mine, Subject: ‘Your Smart Lock with Wi-Fi Bridge Pre-Order Code is Ready!’
My code arrived at 8:57pm on the 30th, I paid $1.99 for it about a month ago.
The price before code is $219.99, the code took off $61.99, making it $158. I had $11.58 in tax bringing it to the final price I stated.
It had this link attached: https://www.amazon.com/dp/B08HYTGFLR/ref=cm_sw_r_cp_api_glc_fabc_Bgy7Fb2YX0JYV
You may need to make an extension for the indicator so that it stays next to the stationary side of the sensor. This will allow you to crack the window. You could also give it play and add a stop on the stationary side when you close the window completely. If you add a magnet to the stopper and the window side of the sensor it will hold the window sensor until all of the slack/play is out of the window side sensor.
https://www.amazon.com/WINSOON-Sliding-Adjustable-Double-Hardware/dp/B073DWFV1G
If you can't get one from Eufy, you can buy a generic adapter on Amazon. The power supply cord to the Homebase 2 is rated at 100-240 VAC to 12 VDC at 2 Amps. The Homebase end of the cable is a 2.1mm male plug. Note that the center pin of the plug needs to be the + connection.
Here's a link to a generic adapter from Amazon that fits those specs.