Use blank vouchers. Since you will be filing a Single tax return for 2020, do not include your spouses name on the voucher at all. Enter your name and SSN. You can download the blank forms from the IRS: https://www.irs.gov/pub/irs-pdf/f1040es.pdf
You can avoid the vouchers entirely by using IRS direct pay. Enter your information only when you set up the payment from your bank: https://www.irs.gov/payments/direct-pay
CORRECTION: I misread the date of death. You will still file married filing jointly for 2020 since your spouse died in 2020 not in 2019. I don't think it matters whether the estimated tax payments have both spouses names on them or just your name.
