Looking for advice on building a catio without too much expenditure?

Me (25F) and my Mum (55F) are new to frugality and are trying to manage our money better after a lifetime of wastefulness. We have two beautiful cats, who are exclusively indoor cats but keep begging to be outside. After losing three cats in car accidents in the local area before, we are trying to be responsible cat parents and keep them safe.

To this end we ideally want a catio which would attach to the house, but the kits you can buy online are upwards of £400, which seems an awful lot. Does anyone have any tips on how we could go about building our own, or how we can repurpose something else second hand that would allow our cats to spend time outdoors this summer?

We have basic DIY skills and power tools, and are willing to learn new skills.