I like living on the edge.
With your level of posts on askaboutmoney you are obviously financially astute. It would be to your advantage to register for ROS. Filling in the form 11 online would not be difficult for you. I have learned a lot about the workings of the tax system from my years of ROS usage. It can be very educational. Also it seems to operate more efficiently than myaccount.
You need to be careul that you're not caught out regardless of whether you have the receipt or not when asked for it. You cannot deem a pension lump sum to be for the prior year if you haven't even made the payment yet. Revenue are very specific about this. See Paragraph 3.4 of the Pensions Manual
If a contribution is paid after the end of the year, but before 31 October of the following year, relief may be claimed for the previous year provided an election to do so is made by the individual on or before 31 October of the following year. As the payment of a qualifying contribution is a pre-condition to the availability of relief, an election cannot be made in advance of such a payment.